Impedance matching system



Feb@ is 39%@ P: H'ISMWH .IMPEDANCE MATCHING SYSTEM Filed OG'ZL. 1,4, 1944 MMM n :Jar

V/v VENTO@ H. SMV 71H /w' T' TOR/VEV Patented Feb. 7, 1950 nach UNITED STATES PATENT OFFICE 2,496,643 IMPEDANCEMATCHING SYSTEM Phillip H.- smith, Fairhaven, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Applicationctober 14, 1944, Serial No. 558,696

i Claims.

This invention relates to impedance matching lguide impedsystems and particularly to wave ance matching systems.

-The copending application of C. C. Cutler, Se-

rial No. 546,687, led July 26, 1944, Patent No.

end of the main wave guide through a wave guide cavity comprising a pair of short branch wave guides connected in parallel.

As used herein, the term complex impedance An adjustable threaded short plug of relatively large di- (Cl. Z50-33.63)

IFI

denotes an impedance comprising resistance and capacitive (positive) or inductive`(negative) reactance. In practice it has been found that optimum or complete matching is not always secured by means ofthe single plug adjustment. Moreover, it is often necessary,- in-a'ddition to adjusting the plug, to change'the dimensions of antenna system described above, the complex in-l put impedance of the branch guides to the main guide impedance without changing the dimensions of the antenna head.

It is one object of this invention to match, in a system comprising a load connected to a main wave guide and having a complex input impedance, the characteristic impedance iof the mainr guide and the complex impedance of the. load.

It is another object of this inventionto matchv the complex impedance of an antenna structure comprising a pair of `open ended branch wave guide sections to the characteristic impedance of a main guide connected to the branch mainv above and also extends an adjustable short distance along the axis of the main guide. Considered from one standpoint, the capacitance of the short plug is in effect connected in shunt with the junction impedance of the two branch wave guides. On the other hand the variable length section of wave guide enclosing the long rod is connected in series with the main wave guide. i

The characteristic impedance of the aforementioned variable section is, by reason of the presence of the` long rod, lower than the characteristic impedance of the main wave guide. In other words, the plug is a capacitive shunt impedance transformer; and the portion of the main guide enclosing the long rod is a tandem or series impedance transformer having a smaller characteristic impedance than that of the main guide.

The shunt impedance transformer functions to transform the complex input impedance of the two-branch guides into another complex impedance .which is transformed by the series impedance transformer. into the characteristic impedance of the main guide. For a given complex load impedance and a given relatively large characteristic impedance of the main guide, the plug and the rod are each adjusted to effect the desired complete impedance transformation. The band-width characteristic of the wave guide system or head is not appreciably changed by the addition of the probe.

The invention will be more fully understood from a perusal of the following specification taken in conjunction with the drawing on which like reference characters denote elements of similar r f function and on which:

Figs. l and 2 are, respectively, top and side j sectional views of one embodiment of the invention; and

Figs. 3 and 4 are, respectively, schematic and impedance diagrams used in explaining the invention.

Referring to Figs. 1 and 2, reference numeral l denotes a translation device such as a radar transceiver and numeral 2 designates a wave guide comprising the straight sections 3 and 4 and the intermediate tapered section 5. The

tapered section 5 functions to match the characteristic impedance Zoo of the wide section 3' to the characteristic impedance Zoi of the narrow secvtion 4. VNumeral 6 denotes an antenna head connected to the open end 'l of main guide section 4 Y and comprising the metallic plate members 8, 9, l0 and l l. The plate members are held together bymeans of the bolts I2 and nuts I3. Plates 8, '9 andv l0 are apertured so as to form two open- J ended short branch wave guides I4 and I5 which are connected to the open end 1 of the main guide 4. The branch guides I4, I5 are connected in parallel and the two open ends I6, I1 of these branch guides constitute transceiving apertures. Numeral I8 denotes a relatively short threaded plug having a large diameter and extending into the head at the junction I 9, Fig. 1, ofthe main guide 4 and the branch guides I4 and I5. As 1 previously indicated, the capacitive plug is in effect connected in shunt to the branch guides at the junction I9 and constitutes a shunt imped ance transformer. Numeral 26 denotes a nut for locking the adjustable plug rI8 in position. AAs-fdescribed so far the structure is substantially the same as the dual aperture rear feed or head ds- '-1 closed and claimed in the above-mentionedco- 1 pending application of C. C. Cutler.

Reference numeral 2l denotes a relatively long threaded rod having a small diameter and extending, in accordance with the invention; eo axially through plug I 3, and, for a short distance, coaxiallyalong the axis 22 rofthe main.;

guide` 4. The coaxial wave guide section 23, Fig. 2, comprising the rod 2l and the portion of guide 4 enclosing the rod constitutes in effect a distributed series impedance transformer having a .characteristic impedance Zoz which is lower than the characteristic impedance Z111 of the .Y

main guide 4. locking the adjustable rod 2I in position.

Numeral 24 denotes a nut for In operation, assuming device I is a transmitter, -waves generated in device I are supplied over guide sections 3, 5 and 4, series transformer 23, junction I9, and branch guides I4 and I5 to the antenna apertures I6 and I1. As explained be,

low -in connection with Figs. 3 and 4, the plug-j I8 androd 2l function to match the complex load vimpedance Z1, comprising the two vparallely` input impedances of branch guides I4 and I5,

to'the characteristic impedance Zof of the main 1 guide 4. The resistive impedance Zoi Vis relatively large-as compared to the maximum value of the resistive component'of the complex pedance Z1. y

Referring tothe schematic diagramof Fig. 3, reference numerals 25,' 26 and 21 denote, respectively, transmissionlines corresponding to the'A guides 4, I4 and i5 of Figs. 1 and 2. Numerals 's 28, 29 and 3G denote, respectively, a junction, a shuntimpedance transformer landy a series impedance transformer corresponding to junction I; I9, the shunt impedance transformer or `capacitive plug IS and the series impedance transforme er 23, of the system of Figs. '1 and'3. The-line sections 3i shown in dotted lines are of negligible length andimpedance, and are shown on the drawing merely for the sake of clarity. The reerence characters Z1, Z2 and Z3 denote, respectively, the input impedance of the load proper comprising lines 26 and 21, the input impedance looking into the junction of lines 26iand 21 and the 'shunt transformer 29, and the input impedance looking into the series transformer 30;v Ref-V erence characters Zoi and Zoz represent the char-A ponent, for any given complex impedance. Since it is desired to match the characteristic impedance Zm of line 25 corresponding to the main guide 4, to the input impedance Z3 of the series transformer 30, or 23 we have Asf'explainedfin the article L-Type Impedance Transforming Circuits by P. H. Smith published in Electronics March 1942, pages 413-52, 54 and 1255-: the range or variation in the impedance Zz,

obtainable by shunting Z2 with a variable capacitive reactance Z4 and varying Z4 over its entire range, maybe represented by the arc 34 of a :circle intersecting the resistance scale 33 at point 35'1and-at the origin 36. Point 35 corresponds to vthemaximum..value of the resistive component of the complex impedance Z1. Reference numeralv31'onthe arc 34 denotes the extremity of a vector 38 having its pole at the origin 36 and representingfathe loadfimpedancev Z1. The arc 34..e^xtends clockwise from point 31 through point 1 35 to the origin 36 and does not include the small arc 136,131.` To determine the particular value of,-Z2, ldenoted :by numeral 39, which may be transformed'by theseries transformer 30 into an input impedanceZ3=Zn1, a circle 4B is drawn which-represents the impedance path followed by the input impedance vector Z3 as the length of yline L1A is varied.- Asexplained in the text book A Microwave Transmission first edition, by J. C. Slater; Chapter 4f1, the circle is centered on the resistance scale 33 andintersects scale 33 at two points, namely, point 4I corresponding to the re- Y sistance4 Z111, and atewhich Z3'=Zoi, and point 42 v The 'L two impedance paths, circles :34 and 40, intersectr 1 each-otheriat pointv 39 coinciding with the ex.A

corresponding toa resistance value of Z5.

tremity of the vector Z2. Reference numeral 43 ,denotes-.the point on the resistance scale 33 corresponding-to theicharacteristic.impedance Zcz of thefslineysection. `'Ihezrelation of points 4I,

429i and .431T corresponding respectively to Z111, Z5. and;fZnz;is,;as stated-in the' Slater text book,

Zorz-:Zoizs (2) Thegradius-A ofthe `circular path 49 and the distanceB between the origin 36 and the center 44"-of :the circle 39 may be determined from the ,following equations acteristic impedances of the main line 25 and theseries transformer 39, respectively.r The clis't A tributed series transformer 36 has an adjustable 1' electrical length denoted by Li/h and the shuntk transformer A29 has an adjustable capacitivere-;.

actance.

Referring to the impedance diagram of Fig. 4, the ordinate or vertical scale 32 represents thel reactance component. ii, and the abscissaor':

horizontal scale `33 yrepresents theresistancecom-g' Assuming for'the moment that Z3 is not real, but complex, its impedance may be represented by .the broken4 line vector 45, the resistive component beingillustrated by the distance C and its `positive reactance by the height D. The impedance Z3 is, however, real in View of Equation 1 so We'have Rez...4 (7) ijX=o (s). K=Z022+R2 (9) Assuming the, complex impedancey Z1 has the .-f1valuegrepresented bywector 38, `thei shunt capaci? f tive transformer 29, Fig. 8, functions to rotate the vector 38 so that its extremity moves along the portion 4E, shown by a zig-zag line, of the arcuate path 34 from point 37 to the point coinciding with the intersection 39 of the circular impedance paths 34 and 4B. The series transformer 33 functions to move the complex impedance Z2 along the portion 41, shown by a zig-Zag line, of the arcuate path 40 to the point 4I on the resistance scale 33. In other words, the shunt transformer 29 changes the complex impedance Z1 into another complex impedance Z2 and the series transformer 30 changes the complex impedance Z2 into an impedance Z3 equal to the characteristic impedance Z111 of the main line 25, whereby the two transformers function to match the load impedance Z1 to the line characteristic impedance Zo1. In this connection it should be noted that, since the maximum value of the resistance component of Z1 is smaller than Za or Z111, two adjustments are required to transform Z1 into Z3, that is, Z1 must be moved along two intersecting circular impedance paths 34 and 49.

In an analogous manner, in the system of Figs. 1 and 2, the capacitive plug I8 transforms the complex input impedance of the open-ended branch guides I4 and I5 into another complex impedance which is transformed by the guide section 23 comprising rod 2I and the adjacent portion of guide 4 into a resistive impedance equal to the characteristic impedance of the main guide 4. While the schematic transmission line system of Fig. 3 may not be exactly comparable in all respects to the wave guide system of Figs. 1 and 2, the systems are believed to be sufficiently analogous for explaining one theory underlying the operation of the wave guide system of Figs. 1 and 2.

Although the invention has been explained in connection with a particular embodiment it is to be understood that it is not to be limited to the embodiment described inasmuch as other apparatus may be employed in successfully practicing the invention.

What is claimed is:

l. A method of matching a relatively large characteristic impedance of a main Wave guide to a relatively small complex input impedance of a pair of open-ended wave guides connected to said main guide and in parallel with each other, utilizing at the junction of said parallel guides and said main guide a pair of independently adjust able cylindrical conductive members coaxially aligned with the axis of the main guide, the rst of said members having a small axial dimension and a large diameter and the second of said members having a large axial dimension and a small diameter, which comprises inserting both members into said parallel guides at said junction and only said second member into said main wave guide, adjusting the projection of said first member into said parallel guides and thereby securing a different input complex impedance, and adjusting the projection of said second member into said main guide and thereby securing a resistive input impedance equal to said characteristic impedance.

2. In combination, a main wave guide, a pair of branch wave guides connected thereto and having a complex input impedance, and means for matching said input impedance and the characteristic impedance of said main guide comprising a conductive linear member extending through the junction of said branch guides and coaxially into said rst main guide.

3. A combination in accordance with claim 2, the axial dimension of said member being greater than its largest cross-sectional dimension.

4. In combination, a source, a main wave guide connected thereto and having a relatively large characteristic impedance, a pair of branch wave guides connected to said main Wave guide and each having at its other end an antenna aperture, a capacitive adjustable metallic plug projecting into said branch guides at their junction and a linear adjustable metallic rod extending coaxially through said plug and coaxially into said main guide, said rod and the enclosing portion of said main guide forming a section of Wave guide, said section being included between said main guide and said junction and having a relatively low characteristic impedance.

5. In combination, a source, a rst wave guide section connected thereto and having a relatively large characteristic impedance, a second wave guide section connected to said rst section and havinga relatively small characteristic impedance, a pair of parallel wave guides connected to said second section and having a complex input impedance, means projecting into the junction of said second section and said parallel guides for changing said complex input impedance into a different complex impedance, said second section comprising means for changing the length of said second section, whereby said different complex impedance may be changed to a resistive input impedance equal to the characteristic impedance of said first section.

PHILLIP H. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,927,393 Darbord Sept. 19, 1933 1,929,878 Clavier Oct. 10, 1933 2,184,729 Bailey Dec. 26, 1939 2,184,771 Roosenstein Dec. 26, 1939 2,241,582 Buschbecl; May 13, 1941 2,258,953 Higgins Oct. 14, 1941 2,422,184 Cutler June 17, 1947 2,422,191 Fox June 17, 1947 2,432,093 Fox Dec. 9, 1947 2,438,914 Hansen Apr. 6, 1948 FOREIGN PATENTS Number Country Date 356,545 Italy Feb. 4, 1938 

